1. Field of the Invention
This invention relates to an objective lens for optical apparatuses, and in particular, to an infinity-corrected objective lens which is suitable for a microscope using wave-lengths of approximately 250 nm in a deep-ultraviolet region and has a high numerical aperture and a high magnification.
2. Description of Related Art
It is known that objective lenses for microscopes using wavelengths of approximately 250 nm in a deep-ultraviolet region are roughly divided into three types. The first type lens, as disclosed in each of Japanese Patent Preliminary Publication Nos. Hei 6-242381 and Hei 10-104510, is constructed with only a plurality of lenses made of the same medium (quartz in most cases) and cannot be in principle corrected for chromatic aberration. The second type lens, as disclosed in each of Japanese Patent Preliminary Publication Nos. Hei 5-72482, Hei 9-243923, and Hei 11-249025, is designed so that lenses made of different media (quartz and fluorite in most cases) are cemented with adhesives, and thereby chromatic aberration can be corrected. The third type lens, recently disclosed in Japenese Patent Preliminary Publication No. Hei 11-167067, is such that lenses made of quartz and fluorite are used to correct chromatic aberration, but they are not cemented with adhesives.
However, the first type lens, which cannot be in principle corrected for chromatic aberration, has the problem that when a light source with a wavelength width (such as a lamp or an excimer laser in which wavelengths are not in a narrow band) is used, light-collecting performance is considerably degraded due to chromatic aberration and thus predetermined resolution governed by the wavelength and the numerical aperture (NA) of the objective lens is not obtained. The second type lens, in which chromatic aberration can be corrected, has no such a problem. However, it has the problem that an adhesive for favorably transmitting deep-ultraviolet light is not virtually available, and even if it is available, difficulties about cementing force and workability are raised. It is common practice to render light, for example, from a lamp, incident on an objective lens, but if light with high energy, for example, from a laser, is rendered incident thereon, this causes the problem that the adhesive is deteriorated by the deep-ultraviolet light and the transmittance of the objective lens is reduced.
The third type lens is designed to solve all the problems encountered by the first and second type lenses. However, the lens discussed in Hei 11-167067 is basically related to an objective lens for laser repair using a deep-ultraviolet laser, and its embodiment discloses only an objective lens with a numerical aperture of 0.4. With this construction, the problem arises that it is quite impossible to reduce the wavelength so that high resolution can be obtained. Specifically, the resolution of a microscope is basically governed by the wavelength and the numerical aperture of the objective lens, but the central wavelength of visible light used in an ordinary microscope is nearly 550 nm and a dry objective lens has a maximum numerical aperture of about 0.9. Thus, if a wavelength used is set at approximately 250 nm, the wavelength is about halved and hence the resolution is approximately doubled. In this case, however, the numerical aperture is the same. In the objective lens with a numerical aperture of 0.4, even though the wavelength used is set at approximately 250 nm, the wavelength is about halved and the numerical aperture is also about halved. Thus, they are offset, and the result is that the resolution is exactly the same as in a conventional microscope.
It is, therefore, an object of the present invention to provide an objective lens which is favorably used in an optical apparatus employing wavelengths in deep-ultraviolet region and has a high numerical aperture so that chromatic aberration can be corrected by only combining single lenses of different media without using any cemented lens and resolution is dramatically improved to meet a fine design involved in a high density of a semiconductor and a large volume of an optical recording medium.
In order to achieve this object, the objective lens of the present invention includes a first lens unit constructed with a plurality of single lenses, having a negative power as a whole, and a second lens unit constructed with a plurality of single lenses, arranged on the object side of the first lens unit. Each of the first and second lens units is provided with air spaces between positive and negative lenses of different media and has at least one pair of lenses designed to satisfy all the following conditions when the parfocal distance of the objective lens is denoted by L (mm), an air apace by d (mm), the radius of curvature of a lens surface with a positive power opposite to the air space by Rp, and the radius of curvature of a lens surface with a negative power opposite to the air space by Rn, and in addition to the pair of lenses, the second lens unit has at least one single positive biconvex lens and at least one single positive meniscus lens, and thereby it becomes possible to correct chromatic aberration and to favorably obtain resolution corresponding to a high numerical aperture and a wavelength used:
0.01 less than dxe2x80x83xe2x80x83(1) 
d/L less than 0.01xe2x80x83xe2x80x83(2) 
0.88 less than Rp/Rn less than 1.14xe2x80x83xe2x80x83(3) 
In the objective lens mentioned above, the second lens unit has at least three pairs of lenses and is designed to satisfy the following condition, in addition to Conditions (1), (2), and (3), when the Abbe""s number of each of positive lenses contained in these pairs of lenses is represented by xcexd dp and the Abbe""s number of each of negative lenses contained in the pairs of lenses is represented by xcexd dn, and thereby the three pairs of lenses become equivalent to at least three cemented lenses so that aberrations including chromatic aberration can be favorably corrected:
xcexd dp greater than xcexd dn xe2x80x83xe2x80x83(4) 
Further, in the objective lens mentioned above, when the second lens unit is designed to have at least one lens configuration in which three adjacent lenses make two pairs of lenses, at least one false pair of lenses corresponding to a cemented triplet is obtained, and it becomes possible to correct chromatic aberration more favorably.
This and other objects as well as features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.